In growing apples, a young apple is enclosed with a bag which provides a light-shielded condition during its growing period, and, when the fruit is sufficiently grown, the bag is removed to expose the fruit to sunlight for the coloring of its surface. In removing the bag, sudden exposure to sunlight after the bag removal causes a so-called "sunburnt" surface of the fruit, reducing the commercial value of the fruit to zero. For this reason, the fruit bag has traditionally been constructed in a double-layer or occasionally as a triple-layer structure. When it is time to remove the bag, only the outer bag (and the intermediate bag in case of a triple bag structure) is removed initially. The fruit remains covered by the innermost bag which has a suitable light permeability rate for several days so that the fruit is gradually introduced to sunlight to prevent sunburning.
The paper used for the above-described double-layer and triple-layer constructed bags is given the additional role of protecting the fruit from insects and bacteria. Protecting the fruit from bacteria greatly reduces the deterioration of fruit and improves the texture of fruit. In particular, paper with black coating on the interior side is used for the outer bag and wax paper (paraffin paper) is used for the inner bag. The inner bag is colored with a suitable color to ensure a certain light permeability rate.
The use of multiple-layer constructed bags to prevent sunburning as described above always requires two separate bag removing operations, once for the outer bag or bags and then for the inner bag. Furthermore, such operations must be conducted totally manually for each fruit, resulting in a considerable burden on the apple farmer.
As a measure to solve the above-described problem, a method was introduced to adopt a double-bag structure consisting of an inner bag colored with a dye or a pigment which fades in sunlight and yet ensures a suitable light shielding rate, and an outer bag with the proper light shielding rate which will realize the inner bag's fading under sunlight throughout the bag-wearing period of a fruit so that only one bag removing operation will be required.
U.S. Pat. No. 5,406,746 teaches a protective bag applied to fruit growing on a plant for protecting the fruit from excessive irradiation by sunlight, disease, insects, and birds without the requirement of two separate bag removing operations. The bag is colored with a coloring agent which has a property to gradually lower its sunlight shielding characteristic in response to irradiation by sunlight. The method is not practical, however, because of the complexity of manufacturing such a bag, including the formulation of the pigment used for controlled fading. It has been found by the inventor of the '746 bag, which is the same inventor of the present invention, that the protective bag of '746 did not satisfactorily succeed in preventing sunburn. Although it is not completely understood, it appears that the degree of permeation of light and light shielding rate characteristic of the '746 bag is unsatisfactory in preventing sunburn to the fruit.